Ever since the advent of cinema, the human race has become more infatuated and intrigued by it and it has become more and more an integral part of our culture. In an age where the boundaries of realism are being pushed in cinema through the use of new technologies such as high definition and 3D, the film industry is constantly trying to create a more believable illusion of realism in film. If we strip the premise of cinema down to its most basic form of that of a series of images in sequence creating the illusion of a motion picture we can explore how we perceive this illusion and how it gave birth to the medium of cinema.

One of the illusions still used in cinema today is the phi phenomenon. This was first seen in Victorian times with the popularisation of a toy called a thaumatrope. This consisted of a disc or card with a picture on each side was attached to two pieces of string. When the strings are twirled quickly between the fingers the card rotates momentarily showing each of the two images. The two pictures appear to combine into a single image. This toy was said to be invented by John Ayrton Paris who in 1824 used it in a demonstration to the Royal College of Physicians in London the idea of persistence of vision. Persistence of vision said that there was a processing delay in the brain and when two images were flashed in front of the eye fast enough the delay caused the first image to remain in place for a fraction of a second longer causing it blend into the second one (Bazin 1967).

In 1912 Max Wertheimer in his book “Experimental Studies on the Seeing of Motion” exposed this theory of persistence of vision as a myth. In its place Wertheimer devised the phi phenomenon along with beta movement help to describe the illusions of motion perception. Wertheimer was one of the founding members of Gestalt psychology and this discovery was a big breakthrough for this field of psychology.

The phi phenomenon discovered by Wertheimer is a perceptual illusion where the viewers mind fills in shapes to help link two images together. In his experiment Wertheimer shows an audience a series of two images. The first image is a line on the left side of the screen. The second image shows a line of the right side of the screen. At different timings Wertheimer observed that the viewers reported a sensation of motion in the space between and around the two lines. However the lines themselves did not appear to move, they simply saw what looked like two moving shapes of the background colour surrounding the flashing lines.

Beta movement was described by Wertheimer (1912) as apparent motion. To prove his theory of beta movement Wertheimer preformed an experiment. He sat viewers in front of a screen where he then projected a two image sequence with varying gaps between them. The first image of a ball on the left of the screen was projected onto the screen followed by a second image with the ball on the right of the screen. The viewers described how they saw the ball move from one side of the screen to the other when in fact they only had seen two still images. This showed that the illusion of movement was created through the perception of the viewers linking the two images together according to one or more of Gestalt’s laws, such as the laws similarity and proximity.

Another way we perceive motion is through stroboscopic images. Leighton (2008) explains that stroboscopic images are a series of static images that when viewed in sequence at a high enough rate; the static images appear to blend into one continuous motion. A simple demonstration of this is the idea of a flip book. If you have a series of pages with simple drawings on them and the drawings are slightly altering from one page to the next and we flip through these pages at speed we perceive them as one continuous picture in motion. However if you flick though the book slowly enough the eye isn’t fooled by this illusion and perceives them as separate images.

We must then ask at what point our eye sees this series of images as motion and not just still images. According to Cubitt (2005) critical fusion frequency is the rate at which stimuli can be presented and still be perceived as separate stimuli. To work stroboscopic images have to present visual stimuli such as images to the eye at a higher rate than the critical fusion frequency so that the eye cannot perceive each individual image separately. The critical fusion frequency differs from person to person and depends heavily upon many conditions such as light and contrast but as a general rule it is said to be anything less than a 7th of a second. This means that if we look at a series of images that are playing at anything above 10 images or frames per second we will see then as one continuous motion.

From when the first commercial movie cameras were manufactured there was no standard frame rate of the cameras, they ranged from anything from 10 frames a second. This was seen unsuitable for cinema as the motion in the film was very jerky. By the 1920’s a standard frame rate of 24 frames a second was set (Bazin 1967). This allowed the illusion of motion in film to be perceived by the viewer to be a lot more smooth and fluid.

Through the use of stroboscopic images, the phi phenomenon and beta movement discussed here modern cinema exploits the visual perception of the viewer to create the illusion of motion which forms the bases for all films. As advancing technologies aid this process of illusion they stride to help cinema create the most realistic representation of what our eyes see and helps transport the viewer into the world of the film as if they where there.